1. Technical Field
Example embodiments of the present invention relate in general to a wireless communication apparatus and more specifically to a delta-sigma modulator and a transmitter including the same that may be applied to a broadband transmitter apparatus.
2. Related Art
A next generation wireless communication system, such as a third generation partnership project (3GPP), a long term evolution (LTE), and a worldwide interoperability for microwave access (WiMAX) of the Institute of Electrical and Electronics Engineers (IEEE), uses a modulated broadband signal through an orthogonal frequency division multiplexing (OFDM) for high spectrum efficiency.
However, in a system using the OFDM such as the LTE and the WiMAX, etc., in case of using a general linear amplifier for signal transmission, there is a problem that power efficiency is decreased due to a high peak to average power ratio (PAPR).
To solve the above problem, studies for maximizing the power efficiency through a power amplifier, such as a Doherty amplifier, an envelope tracking amplifier, etc., have been conducted.
However, since the Doherty amplifier and the envelope tracking amplifier, etc. have a feature that a non-constant envelope signal is applied as an input, when a signal having the high PAPR is applied as the input of the amplifier, there is a problem that nonlinearity of the amplifier increases. Therefore, the power amplifiers have a limit to increase the efficiency.
To solve the problem of the general power amplifier, a transmitter apparatus using a switching amplifier was proposed.
An input signal of the switching amplifier is limited as an envelope signal having a constant amplitude, and in order to produce the input signal, methods such as an envelope delta sigma modulation (EDSM) and an envelope pulse width modulation, etc., were proposed. The switching amplifier always operates in a saturation region using the above input signal encoder, and guarantees linearity for the non-constant envelope signal, thereby obtaining high switching efficiency.
As described above, a structure of the transmitter apparatus using the modulated input signal is called a Class-S structure.
A conventional Class-S structure transmitter apparatus has a structure that a polar coordinate converter separates an input signal into a phase signal and an envelope signal, converts the envelope signal into a pulse-width modulated signal through a low pass delta-sigma modulation and the phase signal into a phase modulated signal through a phase modulation, after this, a mixer produces a sine wave-type signal having the same amplitude by mixing the pulse-width modulated signal and the phase modulated signal, and then a power amplifier is driven by the sine wave-type signal.
However, the conventional Class-S structure transmitter apparatus has a problem that a signal delay is inevitably generated due to an integrator included in a delta-sigma modulator. The signal delay is generated in proportion to the order of the delta-sigma modulator, and if the signal delay occurs, timings of the phase modulated signal and the pulse-width modulated signal at a time point when the both signals are mixed are not matched. That is, the conventional Class-S structure transmitter apparatus has a problem that timings at a time point when dividing the phase signal and the envelope signal and timings at a time point when mixing the modulated signals thereof are mismatched, and then signal quality grows worse.
Meanwhile, the signal quality of the conventional Class-S structure transmitter apparatus is mostly determined by the delta-sigma modulator, and in particular, shows a remarkable difference according to the order and an output level, etc. of the delta-sigma modulator. Accordingly, to maximize the signal quality of the transmitter apparatus, it is necessary to optimize the order and the output level of the delta-sigma modulator.
Further, in the delta-sigma modulator having the same order and output level, the higher an oversampling ratio (OSR) is, the higher a signal to quantization noise ratio (SQNR) becomes. Accordingly, in order to increase much more the signal quality of the transmitter apparatus, it is necessary to increase the OSR of the delta-sigma modulator.